Rotating an object using script?

You cannot use the '+' operator with with two CFrames check the CFrame operators.

More information about CFrame rotation can be found here.

Secondly since you only want to add rotation you add it to the existing CFrame e.g:-

p.CFrame = p.CFrame * CFrame.Angles(0, math.rad(10), 0)

Lastly you need to include some form of 'wait' in an infinite loop else it will allow other code to run causing studio / Roblox to crash in most cases.You have also missed the end but this may just be an oversight.

Putting this code together:-

-- the part we are rotating 
local p = game.Workspace.Part

while true do
	wait(0.2) -- force the script to wait
	p.CFrame = p.CFrame * CFrame.Angles(0, math.rad(10), 0)
end	


-- a better solution would be to create the CFrame.Angles roblox data constructor and store it in a variable as it does not change


local p = game.Workspace.Part
local cf = CFrame.Angles(0, math.rad(10), 0)

while true do
	wait(0.2) -- force the script to wait
	p.CFrame = p.CFrame * cf 
end	

I hope this helps, please comment if you do not understand how / why this code works.